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What does opening the standard PC serial port do, in electrical terms (i.e. what voltages on which pins)?
For example, the ancient VB6 program which controls an apparatus I am tasked with maintaining toggles .PortOpen
to control some TTL. The connection only used 2 pins (bad solders fell apart), so which pins do I solder to?
The only labels / documentation refer to pins 7 and 9, saying 0V and 5V parenthetically, but does .PortOpen
really just put 5V between RI and RTS?.
As a post script, this isn't the weirdest thing about the set up. The TTL I referred to above also connects to an instrument via a BNC to DB9 (!), with only 1 pin used. I guess there was an assumption about a common ground, since the BNC shielding isn't connected to the GND pin? The connection is to the instrument's 'foot pedal' pin, it was a way to remotely trigger the device.
Update
According to this page, the DTR and RTS pins can go high when the port is opened. If they were so configured, they will subsequently go low when the port is closed.
If DTR and RTS are not enabled, opening the port should set both to low (and keep them low).
TTL? Through-the-lens? Transistor-transistor-logic? – None – 2011-06-23T06:49:44.890
The latter. It's a pass through that is opened and closed by a flip-flop toggled by opening/closing the serial port. – reve_etrange – 2011-06-23T06:53:03.687
@reve_etrange: I was asking because serial ports aren't TTL level, so you can't connect them directly. I thought you meant something else. – None – 2011-06-23T06:56:24.853
@Federico Russo, in my embedded applications and debugging I often use TTL level UARTS without drivers. Doesn't everyone have one of these http://www.ftdichip.com/Products/Cables/USBTTLSerial.htm?
– kenny – 2011-06-23T12:47:12.630@kenny: OP says he's on a PC, and there a serial port goes from -12V to +12V. – None – 2011-06-23T15:00:08.180
@Federico Russo, not necessarily, you can find a PC with built-in RS232 any longer. See the link I provided http://ftdichip.com/Products/Cables/USBTTLSerial.htm ?
– kenny – 2011-06-23T15:37:56.630